1. Technical Field
The present disclosure generally relates to absolute position measurement methods and, more particularly, methods for calculating a coarse absolute position by reading a scale pattern obtained by symbolizing a pseudo-random-code and calculating an accurate absolute position using a phase of a code.
The present disclosure also relates to methods for measuring an absolute position and, more particularly, to methods for calculating an absolute position by optically reading a binary scale using an absolute position binary code (APBC).
2. Description of the Related Art
An absolute position encoder allowing a read head to decide an absolute position using an absolute position scale is well known. The absolute position encoder includes a scale to a single track having specific position data that are successively formed along a measuring dimension of the scale. However, resolution limitation of the specific data makes it difficult for the absolute position encoder to provide an accurate position.
An incremental position encoder is an apparatus for measuring a relative position of two objects. The incremental position encoder may recognize the same patterns arranged at regular intervals to provide an accurate relative position. However, the incremental position encoder cannot provide an absolute position.
Accordingly, there is a need for a novel encoder structured to provide an absolute position and an accurate position.
In various precision systems and scientific instruments, accurate position measurements are fundamental components for monitoring and controlling actuating systems. Laser interferometers and optical encoders are typical position sensors. The laser interferometer counts and sub-divides interference fringes to measure a position with a sub-nanometer resolution. A period of the interference fringes is decided by a wavelength of a laser light source.
The optical encoder uses a scale. The scale has uniform and periodical patterns, which has uniform periodic patterns with several or tens of micrometer pitch, and obtains position readouts by processing an interference fringe or an intensity profile.
The laser interferometer can achieve higher accuracy, but requires well-controlled environmental condition and delicate alignment.
In incremental position measurement, the position readout is obtained by accumulating relative displacement from an initial position. The incremental position measurement has been applied to many applications such as precise stage and position monitoring.
However, the incremental position measurement is used to measure only relative displacement and requires initialization using an additional sensor to measure an absolute position.
The absolute position measurement improves efficiency and robustness of precision systems because it does not require initialization and can hand various emergency events without requiring initialization. The absolute position measurement is also advantageous in an application in which power consumption must be strictly controlled.
An absolute encoder requires a specially designed scale. An absolute position binary code (APBC) is encoded to the scale. In the initial stage, the APBC was encoded using a multi-track code and an incremental track is added for high resolution. However, a complex configuration and an alignment issue of an encoder head are unavoidably caused by a multi-track configuration of the scale.
Accordingly, there is a need for a novel encoder structured to provide an absolute position and an accurate position.